1. Field of the Invention
The present invention relates to a microwave antenna using carbon nanotubes as a material for an antenna element, and a process for producing the same. More specifically, the invention relates to a microwave antenna capable of being utilized for high capacity wireless communication, such as UWB (ultra wide band), and a process for producing the same.
2. Description of the Related Art
In the field of communication antennas, various studies have been made with respect to the design of antennas to have a high efficiency, a wide bandwidth and a small size, associated with the enlargement of usable range of radio wave. Among the studies, Douglas Werner, et al. of Pennsylvania University, have theoretically noted that the efficiency is improved by disposing antennas in a fractal form, and the number of antennas can be reduced to ¼. Nathan Cohen of Boston University has noted that the use of the Sierpinski's triangle forms capacitance and inductance owing to the zigzag configuration thereof, so as to enable a wide bandwidth and to reduce the area to ⅙. Both the studies can be referred in Nikkei Science, September 1999, page 10.
A specific form and structure of the fractal antenna is disclosed in WO 97/06578, WO 99/25044and WO 02/01668A2. Specifically, in WO 02/01668A2, a wide bandwidth of from 1 GHz to 9.5 GHz as an area of equal to or lower than −10 dB under an evaluation of reflection coefficient is concretely disclosed.
However, there is a limitation in reduction in size even in a fractal antenna since it contains a polygon corresponding to a particular wavelength and the limited repetition of similar figures thereof. Due to the limitation in size, a strict fractal form are not achieved, thus there is a deviation from the ideal state in the low frequency range. In order to solve the problem, a multilevel antenna having plural polygons combined with each other has been proposed (as shown in JP-T-2003-510871), but it is still insufficient to cope with further demands including small size and wide bandwidth.
A carbon nanotube (CNT) has been considered to be applied various applications owing to the peculiar shape and characteristics thereof. A carbon nanotube has a tubular shape with one-dimensional nature formed by winding a graphene sheet constituted by six-membered rings of carbon atoms. A carbon nanotube having a structure containing one graphene sheet is referred to as a single wall nanotube (SWNT), and a carbon nanotube having a multi-layer structure is referred to as a multi-wall nanotube (MWNT). An SWNT has a diameter of about 1 nm, and an MWNT has a diameter of several tens of nanometer, which are extremely thinner than conventional products that are referred to as carbon fibers.
A carbon nanotube has such a characteristic feature that it has a length in micrometer order and a significantly large aspect ratio of the length with respect to the diameter. Furthermore, a carbon nanotube is a substance that has such an extremely rare characteristic nature that it has both properties of metal and semiconductor owing to the spiral structure of the arrangement of six-membered rings of carbon atoms. Moreover, a carbon nanotube has an extremely high electroconductivity and can flow an electric current of 100 MA/cm2 or more in terms of an electric current density.
A carbon nanotube also has excellent features in mechanical properties in addition to the electric characteristics. That is, it is constituted only carbon atoms, and therefore, it has a Young's modulus exceeding 1 TPa and is considerably tough in spite of the significant light weight thereof. Furthermore, it is a cage material and is rich in elasticity and restoration property. Thus, a carbon nanotube is extremely attractive as an industrial material owing to such various excellent properties.
Such a method for utilizing the excellent characteristics of a carbon nanotube has been developed that carbon nanotubes are formed into a film and applied to wiring and patterning. For example, pattern formation of carbon nanotubes has been made by utilizing the screen printing process and the photolithography technique. These techniques are excellent from the standpoint that a large area can be patterned at a time and are used for patterning an electron source of a field emission display (FED). In these techniques, however, carbon nanotubes are simply dispersed in a solvent and then coated, or coated after mixing with a binder, and therefore, they are insufficient in performance, such as mechanical strength and electroconductivity. Accordingly, they are difficult to be applied to an electrode and an electric circuit.
An example of applying the techniques to an antenna is disclosed in JP-A-2002-7992. However, the antenna having the constitution disclosed in the document is an antenna in coiled form since it is a simple electromagnetic induction type, but the bandwidth thereof is about from −20% to +20% of the designed frequency, and it has not been considered in bandwidth characteristics in the microwave region.
JP-T -2002-503204 discloses that carbon nanotube having a three-dimensional structure can be formed by using functionalized carbon nanotubes. JP-T-2002-503204 discloses such a material to be used simply as a flow cell electrode for chromatography that carbon nanotubes having functional groups bonded thereto for separation and adsorption of substances to be passed thereto are accumulated on a metallic mesh to form a porous material, and such a material that is formed by bonding carbon nanotubes by using an alkoxide of aluminum or silica (the alkoxide itself is an insulating material) as a crosslinking agent. However, JP-T-2002-503204 fails to refer to an application to an antenna.